1. Field of the Invention
This invention relates to leaching processes and, more particularly, to improved processes for leaching minerals that tend to form reaction product layers by altering the reaction product layers via additions of particulate modifiers.
2. The Prior Art
It is recognized by mineral and material scientists that the availability of high grade ore deposits continues to decrease as a function of time; therefore, society must rely upon obtaining valuable constituents of ores from low-grade deposits. Low-grade ore deposits present formidable tasks for economical processing and recovery of metallic values. During the past few decades various hydrometallurgical processes for recovery of metal values from sulfide concentrates have been proposed, studied, and in some cases, plants built and operated. Development of hydrometallurgical processes for handling sulfide concentrates has been to provide alternate technology for conventional smelting operations since the latter results in elaborate, and frequently, expensive particle and gas-cleaning systems designed to reduce pollutants to levels environmentally acceptable to society from ethical, moral and lawful points of view.
Environmental acceptance in conjunction with the rising costs of energy and the necessity to process low-grade ores that cannot be economically upgraded by conventional milling, concentration, and smelting techniques, have opened the door to development of hydrometallurgical alternatives to compete with the traditional methods of metal recovery. Accordingly, hydrometallurgical processes are playing an increasingly important role in the development of new technology for processing certain concentrates.
There are several hydrometallurgical routes to recover metal values from mineral concentrates with the result that these alternate routes are potentially competitive with conventional smelters and offer the possibility of direct recovery of sulfur in the solid state.
One of the most critical steps in the hydrometallurgical operations is the dissolution of the subject metal value. Hence, control of the dissolution process is of central and utmost concern to those involved in leaching operations, in practice or in research. To the scientists and engineers utilizing hydrometallurgical systems it is well-known that the kinetics of dissolution may be controlled by:
a. Transport of reactant(s) and/or product(s) to or from the reaction interface; PA1 b. Chemical reaction(s) at the interface; and PA1 c. Mixed control of (a) and (b) when these processes exhibit comparable kinetics.
Generally, although certainly not always, the dissolution of most oxide minerals is controlled by the reaction at the solid-solution interface (point b) while sulfide dissolution processes are more complex and are frequently found to be controlled by transport processes (point a) especially when a product layer forms at the mineral-solution interface.
Historically, the use of acid ferric sulfate solution as a lixiviant for chalcopyrite concentrates has been of little interest from the practical standpoint due to the formation of a relatively tenacious layer of elemental sulfur which severely impedes the reaction progress. This reaction is as follows: EQU CuFeS.sub.2 +4Fe.sup.+3 .fwdarw.Cu.sup.+2 +5Fe.sup.+2 +2S.degree.(1)
By way of example, an acid ferric sulfate leach of chalcopyrite concentrate conventionally resulted in only 12 percent copper extraction in a three hour leach at 93.degree. C.
Previous experimental results indicated that particle size was the only controllable variable which had a significant effect on the rate of acid ferric sulfate leaching at ambient pressure. A more thorough discussion of the foregoing can be found in Chapter 31 "Acid Ferric Sulfate Leaching of Attritor-Ground Chalcopyrite Concentrates," Extractive Metallurgy of Copper, Vol. II, p. 611 (1976), editors, J. C. Yannopoulos and J. C. Agarwal, an International Symposium sponsored by The Metallurgical Society of AIME, New York, New York.
Attention is also directed to United States Letters Pat. No. 4,115,221 issued Sept. 19, 1978 for ACID FERRIC SULFATE LEACHING OF COPPER SULFIDE CONCENTRATES. This patent discloses a novel process for removing copper from copper sulfide-bearing materials. The copper sulfide-bearing materials are ground to a particle size of at most 1 micron and leached in an acidic solution containing a stoichiometric amount of ferric ions. The ferric ions oxidize the copper and form a pregnant leach solution containing copper ions and ferrous ions.
Additional discussion of the foregoing technique can be found in "Reaction Mechanism for the Acid Ferric Sulfate Leaching of Chalcopyrite," P. B. Munoz, J. D. Miller, and M. E. Wadsworth, METALLURGICAL TRANSACTIONS, Vol. 10B, pages 149-158 (June 1979).
Each of the foregoing references relates to the acid ferric sulfate leaching of chalcopyrite with particular emphasis directed toward overcoming the difficulty encountered upon formation of an elemental sulfur layer on the chalcopyrite surface. Importantly, the elemental sulfur layer on the chalcopyrite surface significantly influences the reaction kinetics by establishing a diffusion barrier. While the details of rate control for the particular reaction have not been well established, several theories have been proposed including, for example, the following: (a) Surface reaction--Under certain circumstances, for example, high anodic potentials, it appears that the elemental sulfur does not form a protective barrier and the reaction may be controlled by a surface reaction. (b) Transport in the chalcopyrite lattice or through the elemental surface reaction product layer. When the elemental sulfur reaction product does appear to form a diffusion barrier and the rate becomes limited by transport through this layer, rather large activation energies have been observed. The evaluation of evidence supporting each of these positions has led to divergent views regarding the nature of the rate-limiting step in the reaction sequence.
An electowinning process has been developed whereby a slurry of mineral concentrate is subjected to a leaching process by an oxidant generated and regenerated at the anode while metal values are recovered at the cathode. However, the dissolution of the mineral is still a critical step particularly for those minerals that form a reaction product layer on the surface of the mineral particle.
Another investigation relating to the ferric sulfate leaching of chalcopyrite is set forth in the publication "Electrochemistry in Silver Catalyzed Ferric Sulfate Leaching of Chalcopyrite," J. D. Miller, P. J. McDonough, and H. Q. Portillo, Process and Fundamental Aspects of Selected Hydrometallurgical Systems, M. C. Kuhn, editor, SME/AIME (1981). In this study, an enhanced rate of leaching was found to be due to the formation of an intermediate silver sulfide (Ag.sub.2 S) film which forms on the CuFeS.sub.2 surface by an exchange reaction. Under these conditions, unlike the uncatalyzed ferric sulfate leach, the elemental sulfur forms a nonprotective reaction product on the silver sulfide crystallites. As a result, the rate appears to become limited by an intermediate electrochemical reaction in the silver sulfide film rather than by transport through the elemental sulfur reaction product.
In addition to the foregoing, the ammonia oxidation leaching of copper sulfides has been studied and has been reduced to practice on at least two occasions. The chemistry of the oxidation reaction is complex and is influenced by temperature, oxidant concentration, ammonia concentration, ammonium salts, and metal amines. For copper sulfides containing iron, the overall rate of reaction is further complicated by the fact that iron is oxidized to the ferric form and precipitated as hematite, (ferric oxide, Fe.sub.2 O.sub.3) which, under certain circumstances, also alters the kinetics of the reaction. For example, see "Ammonia Oxidation Leaching of Chalcopyrite," L. W. Beckstead and J. D. Miller, Fundamental Aspects of Hydrometallurgical Processes, AIChE Symposium Series, the American Institute of Chemical Engineers, 00658812-78-1159-0173, pages 28-40 (1978).
In view of the foregoing, it would be an improvement in the art to provide a process for enhancing the dissolution of minerals wherein the dissolution of the minerals is characterized by the formation of a reaction product layer about the particles of mineral during the leaching process. It would be an even further advancement in the art to provide a novel process for altering the chemical characteristics of the reaction product layer thereby significantly changing the dissolution rate of the mineral. Another advancement in the art would be to provide a process for including particles having known characteristics within the layer of reaction product. Such a novel process is disclosed and claimed herein.